1. Field of the Invention
This invention relates to disk drive suspensions, and more particularly, to improvements in the manufacture of disk drive suspension interconnects to secure better electrical grounding of suspension components such as copper circuit layers to grounded portions of the suspension such as stainless steel layers including stainless steel layers per se and copper metallized stainless steel layers, to enable increased numbers of copper circuit layers, and further relates to suspension products thus manufactured. The invention further relates to a resilient flying lead and flying lead terminus for disk drive suspensions.
2. Description of the Related Art
Disk drive suspension interconnects, such as Integrated Lead Suspensions (ILS) for hard disk drives typically have three layers, including a stainless steel foil that provides mechanical properties for the suspension, two or more conductive traces comprising gold plated, patterned copper conductive circuits paths that provide the electrical connection between the read/write head slider and the termination pads of the suspension, and a dielectric layer that provides electrical insulation between the stainless steel foil and the conductive traces.
It is known to be desirable to ground various components of a disk drive suspension such as the body of the read/write head slider. One of the major challenges in the design of hard disk drive suspensions is attaining a suitable, reliable grounding connection between the conductive copper traces connected electrically to the slider and the underlying stainless steel foil layer given the limited space available to make the connection. The difficulty of bonding to stainless steel and dissimilarity of the metals (Cu, Au, SST) pose additional significant challenges to creating a reliable grounding of the slider, but reliable grounding is essential to maintaining the signal fidelity between the read/write head and amplifier.
Among the prior art approaches to solving the slider grounding problem is creating a hole in the dielectric between the slider and the stainless steel foil, typically 25 μm deep, and filling the hole with conductive adhesive to provide an electrical connection between the slider and the stainless steel. This approach is deficient, however, since conductive adhesive connections are typified by very high interconnect resistance resultant from the passive (self-healing) nature of the stainless steel and the lack of a conductive, fully metallic bond between the steel layer and the conductive adhesive. High interconnect resistance limits the quality of the electrical connection to ground and thus slider performance dependent on a good grounding is degraded.
Another approach to slider grounding uses a spanning lead that extends from the slider, beyond the edge of the dielectric layer and opposite the stainless steel layer where it is subsequently bent over onto the stainless steel layer and electrically and mechanically affixed there, using, typically, a conductive polymer. Spanning leads are very fragile and can be easily mis-bent during manufacture causing lowered manufacturing yields. Further, even if perfectly accomplished, the process of physically bending and adhering leads to the stainless steel suspensions is a very labor-intensive process that does not lend itself to high-volume, low-cost manufacturing, such as simultaneous gang bonding of multiple suspensions.
In both of these prior art processes the presence of conductive adhesives can cause increased drive contamination that may adversely affect drive reliability, and their use is environmentally undesirable for workers.
Additionally, it is known to provide suspension circuits having tail termination pads at the ends of the circuits, which are flying or unsupported metallic conductors. These structures, are sometimes called flying leads. One purpose of the flying lead region is to allow access to both surfaces of the conductive lead. The flying lead is typically terminated to a rigid or flexible circuit on the suspension actuator using thermosonic bonding. The flying leads have metallic conductors that are unsupported by the dielectric layer that normally separates the conductive signal traces from the other conductive layers and the substrate such as stainless steel below. The flying leads therefore lack the stiffness provided by the underlying dielectric layer. U.S. Patent Publication No. 2005/0254175 by Swanson et al. shows in FIG. 2 a flying lead region 50.
Various constructions and metallurgies have been proposed for the flying leads. Swanson et al. disclose, for example, a first embodiment of a flying lead construction in FIGS. 15A-15C in which a flying lead comprises a copper signal conductor on stainless steel with nickel and gold plating, and a second embodiment in FIGS. 17A-17C in which a flying lead comprises a copper signal conductor with nickel and gold plating. U.S. Patent Publication No. 2007/0041123 by Swanson et al. discloses a flying lead portion formed of a nobel metal. U.S. Pat. No. 5,666,717 issued to Matsumoto discloses in FIGS. 1 and 2 unsupported flying leads formed by cladding (a subtractive process), sputtering, vacuum deposition, or ion plating. Matsumoto employs a conductor metal formed of copper and nickel, and overplated with a nobel metal such as gold which is resistant to corrosion and chemical etching.
During the disk drive manufacturing process, the flying leads can be used for test purposes. U.S. Pat. No. 7,110,222 issued to Erpelding describes integrated lead suspensions and tail pad terminations of those suspensions. The tail pads can be electrically connected via soldering or thermosonic bonding, both of which are widely known and practiced in the microelectronics packaging field. U.S. Patent Publication No. 2005/0254175 by Swanson et al. in FIG. 2 shows a test pad portion 46 on the side of the flying leads away from the suspension. Such a test pad portion is typically used to test the completed suspension assembly. If it is found that a read-write head, also referred to as a slider, on a suspension assembly is defective, the head must be replaced by parting the flexure tail bond and replacing the head. On the other hand if the read/write head passes its tests, typically the test pad portion 46 is cut off as no longer necessary, and the suspension is integrated into a completed disk drive unit. The fragile unsupported lead is prone to damage during assembly, test, or when separating the ultrasonic or solder terminal of this terminus for rework. In recent years, as the thickness of the copper conductor layer has decreased from about 12 μm to about 7 μm in the last few years, the lead has become even more fragile, making rework even more difficult.
Traditional methods of increasing the strength of these delicate unsupported flying leads include the use of copper alloys such as beryllium copper alloy as taught by Bennin et al. in U.S. Pat. Nos. 5,645,735 and 5,687,479, or less toxic copper alloy alternatives such as NK120 or Olin 7025 alloy. Other efforts have focused on methods of distributing the high stress on the terminations at the point of highest strain, where the unsupported leads emerge from the polyimide. An unintended drawback to increasing the toughness of the copper conductor by substituting a stronger copper alloy for use in the suspension signal traces, is that doing so undesirably increases the gimbal stiffness, whereas decreased gimbal stiffness is desired for the emerging smaller read/write heads.
The methods of Swanson et al. and Matsumoto et al. may provide a more rigid tail flying lead terminus than in prior flying leads; however, they require subsequent metal electrodeposition upon dissimilar metals. This is particularly problematic in the case of a copper conductor upon an inert steel, as the presence of the inherent passive amorphous oxide which inherently forms on stainless steel is not readily receptive to acceptable adhesion of subsequent metal deposition.